While night vision goggles may find commercial applications in civilian use, the primary use of such goggles is with the military. Night vision goggles enable a person to see objects in the darkness, which objects could not otherwise be seen by the naked eye. The principle by which night vision goggles operate is that there is generally some source of external light, such as the stars or moon, which are capable of illuminating objects. While the naked eye may not be able to detect such illuminations reflected from an object, the reflections can be electrically amplified so as to be visible to the observer's eye.
Therefore, the principle by which night vision goggles can make objects viewable in darkness is the electrical amplification of reflected light. An image of the object is displayed on a phosphorescent screen within the goggle. A battery powered photomultiplier or image intensifier tube is conventionally used to electrically amplify the light signals for presentation on the green phosphor coated screen. The image is monochromatic, with the intensity of the phosphor representative of the amount of light reflected from the object. The phosphor coated screen is very sensitive and subject to accelerated degradation when exposed to high intensity light. Excessively lighted objects may thus overload the image tube and wash out the display on the phosphor screen.
An on-off switch is typically mounted on the night vision goggle to switch battery power to the image intensifier tube during use, and for removing battery power from the tube either during nonuse to conserve battery power, or during lighted conditions to extend the life of the image intensifier tube. Because night vision goggles may be worn for extended periods of time, headgear is also provided for detachably mounting the goggles thereto, thereby allowing the wearer the free use of both hands. Night vision goggles are commonly detachably mounted to the headgear so as to be quickly removed should an emergency situation arise.
While the foregoing arrangement can greatly facilitate the nocturnal activity of a soldier, there are several inherent shortcomings. For example, if the battery is not switched off after the night vision goggles are removed from the headgear, the battery life can be substantially shortened. Other than roughly guessing at the number of hours of battery use, the soldier has no indication that the battery is nearly discharged. More importantly, if the night vision goggle is removed from the headgear, the green phosphorescent glow emanating from the exposed rear of the goggle can be quickly and easily spotted by others who are also wearing night vision goggles, thereby exposing the wearer to danger. Both the life of the battery and that of the image intensifier tube can be shortened if the goggle power is inadvertently left on during periods of normal lighting.
From the foregoing, it may be seen that a need has arisen for a mechanism which automatically removes the battery power from the night vision goggle when detached from the headgear, or when subjected to periods of normal lighting. Because such automatic power removal mechanism itself utilizes battery power, it is important that the power drain is extremely low. It is also important that the subsequent reengagement of the goggle to the headgear does not automatically reapply power to the goggle without an intentional act to do so by the user. This is useful in field operations where a reengagement of the goggle to the headgear for storage purposes would power up the goggle and cause an unnecessary drain on the battery. There is an associated need to provide a visual indication of the imminent discharged state of the battery.